Rolling element bearing

ABSTRACT

A rolling element bearing having balls and races, each having the surface of rolling contact coated with a laminar solid lubricant, such as molybdenum or tungsten disulfide, and a retainer made of a heat-resistant composite material having self-lubricating properties. The retainer material is composed of a polyether ether ketone resin, a polytetrafluoroethylene resin, one or both of molybdenum and tungsten disulfides, and polyamide fibers. During the initial period of its use, the bearing is lubricated by the MoS 2  with which the surfaces of rolling contact are coated, and as the films of MoS 2  become worn with the passage of time, the frictional contact between the balls and the retainer transfers an appropriate amount of material from the retainer to the surfaces of the balls and, further, to the track surfaces of the races to continuously form lubricating films thereon. The bearing is, therefore, not substantially worn during its use in a vacuum, or in air, but has a long life.

FIELD OF ART

This invention relates to rolling element bearings which are used in avacuum, in an environment having a high or low temperature, in thepresence of radioactive rays, or in any other environment not permittingthe use of any lubricating oil, or grease.

BACKGROUND OF THE INVENTION

There are three known methods for lubricating rolling element bearingsfor machines operating in an environment not permitting the use of anylubricating oil, or grease:

(1) A thin lubricating film of, for example, molybdenum disulfide(MoS₂), tungsten disulfide (WS₂), or silver (Ag) is formed bysputtering, or otherwise, on the sliding surfaces of all or a pert ofthe inner and outer races, the retainer and balls, as typicallydescribed in Japanese Patent Unexamined Publication No. 55-57717,58-113629 or 61-55410;

(2) A retainer is made of a sintered alloy produced by adding a solidlubricant, such as MoS₂ or WS₂, to a metal or alloy, and the retainermaterial is transferred by the friction between the retainer and ballsto the balls, and from the balls to the inner and outer races, to form athin lubricating film on each of the balls, and inner and outer races,as typically described in Japanese Patent Unexamined Publication No.62-151539 or 63-246507; and

(3) A retainer is formed from a plastic material or a compositehigh-molecular material prepared by adding a solid lubricant to theplastic material, and the retainer material is transferred by thefriction between the retainer and balls to the balls, and from the ballsto the inner and outer races, to form a thin lubricating film on each ofthe balls, and inner and outer races. Typical examples are retainersformed from fluororesins known as Rulon E (tradename of NTN Rulon) andDuroid (tradename of Rogers Corp., U.S.A.). These retainers are formedfrom a mixture of a polytetrafluoroethylene resin (hereinafter referredto simply as "PTFE"), glass fiber and MoS₂.

The preliminary formation of thin lubricating films as described in (1)above, however, generally causes a bearing to have a short life, sincethe frictional wear of the films results in the loss of theirlubricating effect and the exposure of the metal as a base material,causing its seizure.

The retainer made of a sintered metal as described in (2) above has thedrawbacks of being difficult to machine, making significant frictionnoise, brittleness, cracking during use, and being heavily worn whenexposed to air.

A bearing including a retainer formed from PTFE reinforced by glassfiber as described in (3) above has the drawbacks of quickly becomingworn, generating so much heat as to be likely to cause a variation oftorque during use, since the glass fibers projecting from the surface ofthe retainer scrape off the lubricant transferred from the retainer androughen the surfaces of the balls.

According to the solution as proposed, for example, in Japanese PatentUnexamined Publication No. 62-261718, hydrofluoric acid is used to meltglass fibers away from the machined surfaces of the retainer.Hydrofluoric acid is, however, toxic and its handling calls for specialcare. Moreover, glass fibers are exposed sooner or later again on theretainer surfaces, because of the wear of PTFE, and exert an adverseeffect on lubrication similar to no such treatment being given at all.

There are also known retainers formed from composite materialscontaining high-molecular materials other than PTFE. A retainer formedform thermally stable polyimides, however, has the drawback of producinga large amount of gas in a vacuum. Japanese Patent Publication No.63-1989 describes a retainer formed from a composition comprising athermoplastic polyether aromatic ketone, a fluororesin and aromaticpolyamide fibers. This retainer, however, has the drawback of beingunevenly worn due to a greatly varying coefficient of friction, andthereby causing a variation of torque and axial deflection.

The known retainers formed form composite high-molecular, or sinteredmaterials get worn more rapidly in their portions on which a load bears,and fail to supply the lubricant uniformly to the inner and outer races,and to the balls. The bearings including such retainers have, therefore,the drawbacks of being unstable in torque, producing a large amount ofdust, and having a short life.

Under these circumstances, it is an object of this invention to providea rolling element bearing which has an improved degree of lubricationowing to the steady and uniform transfer of a lubrication, and has a lowlevel of torque and a long life, produces only a small amount of dustand gas, makes only a slight noise, and is easy and inexpensive tomanufacture.

DISCLOSURE OF THE INVENTION

This invention resides in a rolling element bearing which comprisesballs and races each having the surface of rolling contact coated with alayer of solid lubricant, such as molybdenum disulfide or tungstendisulfide, and a retainer made of a heat-resistant composite materialhaving self-lubricating properties.

The composite material of which the retainer is made comprises apolyether ether ketone resin, a polytetrafluoroethylene resin, one orboth of molybdenum disulfide and tungsten disulfide, and polyamidefibers.

During the initial period of rotation of the bearing, the solidlubricant MoS₂ or WS₂ functions to lubricate the surfaces of the ballsand the races, which are coated with it. As the coating films wear withthe passage of time, the friction between the balls and the retainertransfer the solid lubricant from the retainer to the surfaces of theballs to form thin films of the lubricant which lubricate the balls andthe races, so that the bearing can be steadily lubricated for a longtime.

The polyether ether ketone resin (hereinafter referred to simply as"PEEK") which is used to make the retainer is tough, highly resistant toheat, and self-lubricating, but is difficult to transfer when usedalone. Therefore, PTFE, and one or both of MoS₂ and WS₂, which arelow-friction, low-wear and heat-resistant laminar solid lubricants whichare easy to transfer and do not produce a large amount of gas in avacuum, are added in appropriate amounts with PEEK to facilitate steadytransfer and to form films having improved lubricating properties.

Any reduction in hardness and strength that would result from theaddition of these materials can be prevented by the addition of aheat-resistant organic fibrous reinforcing material.

The reinforcing material is required:

(a) To withstand the temperature at which the PEEK resin is melted formolding;

(b) To exhibit a reinforcing effect in the temperature range in whichthe bearing is used;

(c) To be self-lubricating, so that no fiber projecting from the surfaceof the retainer may scrape thin lubricant films off the balls, orroughen the surfaces of the balls or the track surfaces of the inner andouter races, and thereby hinder the proper lubrication thereof

(d) To avoid producing a large amount of gas in a vacuum;

(e) To well maintain the PEEK resin's moldability and machinability; and

(f) To avoid making any large noise from friction.

Aromatic polyamide, aromatic polyamide-imide (PAI), and polyether-imide(PEI) fibers are examples of heat-resistant organic fibers which satisfythe requirements listed above. Aromatic polyamide (aramid) fibers are,among others, preferred from the standpoints of cost, easy availability,reinforcing effects, ease of handling, etc.

The aramid fibers are self-lubricating, and do not exert any adverseeffect on lubrication as glass fibers do, even if they are exposed on afriction surface. Although PTFE itself shows a greatly varyingcoefficient of friction, the addition of one or both of MoS₂ and WS₂enables the formation and uniform and stable lubricating films showing alower degree of variation in the friction coefficient.

FIG. 5 shows the results of pin-on-disk friction tests conducted oncomposite materials containing a PEEK resin to determine their dynamicfriction coefficients and their wear in a vacuum. A mixture of PEEK,PTFE and aramid fibers showed greatly varying dynamic frictioncoefficients in a vacuum, as shown in (b) in FIG. 5, though its averagevalue lessened with the distance of friction. A mixture furthercontaining an appropriate amount of MoS₂ showed a lower frictioncoefficient with a smaller variation, as shown in (a) in FIG. 5. ThePEEK used alone showed a greater range of friction and a greater degreeof wear, as shown in (c). The examination of the friction surfacesconfirmed that the addition of MoS₂ had been effective, as a thin filmof transferred material had spread uniformly on the whole frictionsurface when the composition shown in (a) was used, while no uniformfilm had been formed when the composition shown in (b) or (c) was used.These results confirmed that the composite material containing MoS₂produced a very small amount of friction, and could make a retainer oflow friction and wear capable of continually forming a thin film bytransfer with any surface containing it. A composite material comprisingPEEK, aramid fibers and MoS₂, and not containing PTFE was found tobecome too heavily worn to be suitable for a retainer.

Although the foregoing describes a case in which MoS₂ is used, it isequally possible to use WS₂, which is also a substance having a laminarstructure, or a mixture of MoS₂ and WS₂.

A retainer can be made if a mixture of various materials as hereinabovementioned is injection, extrusion, or compression molded under theconditions which are usually employed for making a molded product orPEEK.

The PEEK resin and all the other materials used with it to make aretainer produce only a small amount of gas in a vacuum, and itnecessarily follows that the molded product thereof produces only asmall amount of gas.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional view of an angular contact rolling bearingembodying this invention;

FIG. 2 is an enlarged perspective view of the retainer in the bearing ofFIG. 1;

FIG. 3 is a front elevational view, partly in section, of the bearingembodying this invention;

FIG. 4 is a chart showing a variation of torque as observed with thepassage of time on each of the bearings embodying this invention andcomparative examples of bearings, and their life; and

FIGS. 5(a), 5(b), and 5(c) are charts showing the friction coefficientsof retainer materials as determined by a model testing machine.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention will now be described in further detail with reference tothe drawings.

FIG. 1 is a cross sectional view of a solid-lubricated angular contactrolling bearing 1 embodying this invention. A film 6 of MoS₂ having athickness of 0.5 micron was formed by sputtering on each of the tracksurfaces of the inner and outer races 2 and 3 made of stainless steelSUS 440C, and the surfaces of balls 4.

FIG. 2 is a perspective view of a retainer 5. It is identical in shapeto a conventional retainer of the bore type, but was made from differentmaterials by a different process, as will hereunder be described.

The materials listed below were used for making the retainer 5:

    ______________________________________                                        PEEK:      "PEEK" of ICI, England (a powdery                                             material withstanding continuous use at a                                     temperature of 240° C. and having a melting                            point of 334° C.);                                          PTFE:      "Teflon 7J" of Mitsui Du Pont                                                 Fluorochemical Co.;                                                Aramid fibers:                                                                           "Kevlar 49" of Du Pont (chopped strands                                       having a length of 1 mm);                                          MoS.sub.2 :                                                                              "Molykote Microsize" of Dowcorning.                                ______________________________________                                    

These materials were mixed in the weight proportions listed below toprepare a dry mixture:

    ______________________________________                                        (1)       PEEK:       60%                                                     (2)       PTFE:       20%                                                     (3)       Aramid fibers:                                                                            10%                                                     (4)       MoS.sub.2 : 10%                                                     ______________________________________                                    

The mixture was melted, and extruded by an extruder having a compressionratio of 3/1 and a temperature of 350° C. to 380° C. to make a moldedround bar of uniform composition having a diameter of 20 mm. The roundbar was machined to make a retainer of the bore type adapted for fittingan angular contact rolling bearing having an inside diameter of 8 mm,and outside diameter of 22 mm and width of 7 mm.

The retainer, inner and outer races, and balls were put together tofabricate the angular contact rolling bearing 1 as shown in FIG. 1.

A durability test was conducted to determine the torque of the bearingin operation, its life, and the wear of the retainer. The test wasconducted under these conditions:

    ______________________________________                                        Atmospheric Pressure:                                                                          (2 to 4) × 10.sup.-7 torr, open air;                   Thrust load:     7 kgf;                                                       Bearing temperature:                                                                           120° C.;                                              Rotating speed:  1800 rpm.                                                    ______________________________________                                    

For the sake of comparison, similar test were conducted on bearingshaving retainers made of five kinds of different materials as listedbelow:

(A) A sintered Cu-Sn alloy containing 30% WS₂ and MoS₂ ;

(B) A cast alloy comprising 80% Cu, 10% Sn and 10% Pb;

(C) Ceramics containing B₄ C;

(D) An Ekonol resin (aromatic polyester); and

(E) Rulon E (fluororesin).

The results of the tests are shown in FIG. 4 and TABLE 1.

As is obvious from FIG. 4, it is desirable for the torque in operationto be small to reduce any loss at the bearing. The failure of thebearing to be properly lubricated brings about an increase of torque andresults in the seizure of the bearing, resulting in the end of its life.In the chart, L indicates the end of the life of the bearing.

The bearing of this invention has a smaller amount of torque having asmaller variation, and a longer life than any of the bearings in thecomparative examples.

Referring to TABLE 1, `wear` means the worn volume of the retainer perrevolution (1×10⁻⁸ mm³ /rev). The results shown in TABLE 1 confirm thatthe retainer in the bearing of this invention is more resistant to wear,both in a vacuum and exposed to air, than any of the comparativeretainers.

Although the retainer is described as having been made by extrusionmolding and machining, it is also possible to make a retainer byinjection or compression molding if an appropriate mold is prepared.

                  TABLE 1                                                         ______________________________________                                        Wear of Retainers as Determined by Durability Tests                                            Wear                                                         Retainer Materials In a vacuum                                                                              In air                                          ______________________________________                                        Invention          2.84       2.78                                            Comparative Example (A)                                                                          3.02       8.18                                            Comparative Example (B)                                                                          6.17       594.1                                           Comparative Example (C)                                                                          55.1       --                                              Comparative Example (D)                                                                          21.1       --                                              Comparative Example (E)                                                                          --         2.87                                            Comparative Example (F)                                                                          3.48       3.58                                            ______________________________________                                    

The retainer can be formed from a composite material containing 10 to40% by weight of PTFE, 5 to 30% by weight of aramid fibers, and 5 to 30%by weight of MoS₂ or WS₂, the balance thereof being PEEK. The use of anycomposition containing less than 10% by weight of PTFE is undesirable,as it results in a retainer which is low in wear resistance. The use ofany composition containing more than 40% by weight of PTFE is alsoundesirable, a sit results in a retainer of low strength. The use of anycomposition containing less than 5% by weight of aramid fibers resultsin a product of low strength. Any composition containing more than 30%by weight of aramid fibers is too low in flowability to be molded. Theuse of any composition containing less than 5% by weight of MoS₂ or WS₂results in a product of low wear resistance. Finally, the use of anycomposition containing more than 30% by weight of MoS₂ or WS₂ results ina product of low strength.

As is obvious from the foregoing description, the rolling elementbearing of this invention having the surface of rolling contact coatedwith the solid lubricant MoS₂, and a retainer made of a PEEK resincomposition containing PTFE, aramid fibers and MoS₂, and having alubricating action, is properly lubricated by the MoS₂ on the surfacesof rolling contact during the initial period of use, and as the films ofMoS₂ on the surfaces of rolling contact become worn with the passage oftime, the frictional contact between the ball 4 and the retainers 5transfers an appropriate amount of material from the retainer to thesurfaces of the balls and, further, to the track surfaces of the racesto continuously from lubricating films 7 thereon. Therefore, the bearingis not substantially worn during its use in a vacuum, or in air, and hasa long life. Thus, it is maintenance-free over a long period of timebecause of the low amount of friction and the long life, makes only asmall amount of friction noise, produces only a small amount of gas evenin a vacuum, and is inexpensive as compared with any conventionalrolling contact bearing supplied with a solid lubricant.

INDUSTRIAL FEASIBILITY

The bearing of this invention is suitable for use with a conveyingdevice used in a wafer, or other semiconductor manufacturing apparatus(in a vacuum, or at a high temperature), a machine or apparatus used inouter space (in a vacuum, or at a high or low temperature), or anunmanned robot working in facilities involving the production or use ofatomic energy, or any other machine or apparatus used in a vacuum, at ahigh or low temperature, or in the presence of radioactive rays.

We claim:
 1. A rolling element bearing comprising an outer race, aninner race, balls, and a retainer holding said balls in an appropriatelyspaced relation from one another, said retainer being a product of amixture of polytetrafluoroethylene, MoS₂ or WS₂, aramid fibers, and apolyether ether ketone resin.
 2. A bearing as claimed in claim 1,wherein the surfaces of said balls and the track surfaces of said racesare coated with MoS₂ or WS₂.
 3. A bearing as claimed in claim 1, whereinsaid mixture contains 10 to 40% by weight of polytetrafluoroethylene, 5to 30 by weight of aramid fibers, and 5 to 30% by weight of MoS₂, thebalance of its composition being of said polyether ether ketone resin.